2,161 research outputs found
Adiabatic suppression of the axion abundance and isocurvature due to coupling to hidden monopoles
The string theory predicts many light fields called moduli and axions, which
cause a cosmological problem due to the overproduction of their coherent
oscillation after inflation. One of the prominent solutions is an adiabatic
suppression mechanism, which, however, is non-trivial to achieve in the case of
axions because it necessitates a large effective mass term which decreases as a
function of time. The purpose of this paper is twofold. First, we provide an
analytic method to calculate the cosmological abundance of coherent oscillation
in a general situation under the adiabatic suppression mechanism. Secondly, we
apply our method to some concrete examples, including the one where a string
axion acquires a large effective mass due to the Witten effect in the presence
of hidden monopoles.Comment: 30 pages, 3 figure
On Longevity of I-ball/Oscillon
We study I-balls/oscillons, which are long-lived, quasi-periodic, and
spatially localized solutions in real scalar field theories. Contrary to the
case of Q-balls, there is no evident conserved charge that stabilizes the
localized configuration. Nevertheless, in many classical numerical simulations,
it has been shown that they are extremely long-lived. In this paper, we clarify
the reason for the longevity, and show how the exponential separation of time
scales emerges dynamically. Those solutions are time-periodic with a typical
frequency of a mass scale of a scalar field. This observation implies that they
can be understood by the effective theory after integrating out relativistic
modes. We find that the resulting effective theory has an approximate global
U(1) symmetry reflecting an approximate number conservation in the
non-relativistic regime. As a result, the profile of those solutions is
obtained via the bounce method, just like Q-balls, as long as the breaking of
the U(1) symmetry is small enough. We then discuss the decay processes of the
I-ball/oscillon by the breaking of the U(1) symmetry, namely the production of
relativistic modes via number violating processes. We show that the imaginary
part is exponentially suppressed, which explains the extraordinary longevity of
I-ball/oscillon. In addition, we find that there are some attractor behaviors
during the evolution of I-ball/oscillon that further enhance the lifetime. The
validity of our effective theory is confirmed by classical numerical
simulations. Our formalism may also be useful to study condensates of ultra
light bosonic dark matter, such as fuzzy dark matter, and axion stars, for
instance.Comment: 31 pages, 8 figures; v2: typos fixed, published version; v3: typos in
the figures fixe
Suppressing the QCD Axion Abundance by Hidden Monopoles
We study the Witten effect of hidden monopoles on the QCD axion dynamics, and
show that its abundance as well as isocurvature perturbations can be
significantly suppressed if there is a sufficient amount of hidden monopoles.
When the hidden monopoles make up a significant fraction of dark matter, the
Witten effect suppresses the abundance of axion with the decay constant smaller
than GeV. The cosmological domain wall problem of the QCD axion can
also be avoided, relaxing the upper bound on the decay constant when the
Peccei-Quinn symmetry is spontaneously broken after inflation.Comment: 6 pages, 1 figur
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